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Understanding Ash Fusibility (Clinkers)
W
hen it comes to the science of biomass combustion, no parameter is more complicated than ash fusion. When ash fusion characteristics are favorable, ash remains ash, and the only challenge is to remove it from the combustion system in such a way that it does not build up excessively. Conversely, when ash fusion characteristics are not favorable, strange things happen—ash can clump together and may need to be broken up, or even chiseled out of an ash pan. It can form a brick, look like a fused piece of glass, or even resemble a honeycomb. This unwanted ash behavior is referred to as fouling or slagging when it builds up in industrial burners,
BY CHRIS WIBERG and is referred to as clinkers in smaller scale combustion appliances. No matter what you call it or how it looks, the process by which it forms is relatively simple, as it is just a function of melting point. However, troubleshooting it can be complex because there are many factors involved. First, let’s start by qualifying that “clean” ash (free of dirt, rocks, unburned carbon, etc.) is primarily a combination of inorganic oxides. When biomass is burned, the organic material (essentially all the carbon, hydrogen, nitrogen and oxygen) is released, and inorganic minerals remain in the oxidized form that we see as ash. Through testing, we know that biomass ash is primarily composed of oxides of calcium, silica,
aluminum, magnesium, potassium, manganese, sodium, iron, phosphorus and other minerals. Each of these oxidized minerals is present as a solid, and like any other solid, they have a melting point. The range of melting points for the various mineral oxides present can vary greatly, and the overall melting point of the collective ash is a function of all the mineral fractions and chemical interactions that occur at the elevated temperatures of the combustion system. As a result, ash generally melts over a range of temperatures, rather than a specific temperature. The range can be anywhere from a few degrees to 50 or even 100 degrees Celsius. That’s why when you see ash fusion test results, it is reported as
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22 PELLET MILL MAGAZINE | ISSUE 1 2022
